1. Field of the Invention
The present invention generally relates to an acceleration sensor which senses a shock or acceleration applied to an object. More particularly, the present invention is concerned with an acceleration sensor of a piezoelectric type that senses the amount of feature produced by inertia force caused in an object due to an acceleration.
Recently, downsizing of electronic apparatuses has progressed and portable electronic apparatuses such as notebook-type personal computers have been spread. Usually, the portable electronic apparatuses are designed to sense a shock unexpectedly applied thereto and carry out a given process in order to maintain the reliability. For example, if a hard disk drive built in a notebook type personal computer gets a shock, the current write/reproduction operation is interrupted, and rather a process for avoiding occurrence of error is initiated. An acceleration sensor can be used to sense a shock applied to the hard disk drive. The acceleration sensor can also be used to sense a shock in order to promptly activate an airbag apparatus for protecting a passenger of a vehicle from a shock applied thereto due to an accident.
The recent downsized electronic apparatuses require compact, thin, less-expensive acceleration sensors while retaining the original sense performance.
2. Description of the Related Art
Various types of acceleration sensors are known. Recently, an acceleration sensor of a piezoelectric type has begun to attract notice. Japanese Laid-Open Patent Application No. 11-51960 discloses an acceleration sensor that has a piezoelectric element to which a weight element is attached. The acceleration sensor senses acceleration using inertia force caused in the sharing direction between the weight element and the piezoelectric element at the time of receiving a shock.
Japanese Laid-Open Patent Application No. 10-96742 or Japanese Laid-Open Patent Application No. 11-211748 discloses an acceleration sensor which has a piezoelectric element consisting of piezoelectric plates bonded together and senses acceleration from a deflection of the piezoelectric element at the time of vibration.
Japanese Laid-Open Patent Application No. 6-273439 discloses an acceleration sensor which has a cantilever structure of a piezoelectric element consisting of piezoelectric plates bonded together and which senses acceleration from a deflection of the piezoelectric element which is deformed due to inertia force.
However, the conventional types of acceleration sensors mentioned above have the following disadvantages.
The acceleration sensor disclosed in Japanese Laid-Open Patent Application No. 11-51960 or the like needs a troublesome work of attaching the weight element to the piezoelectric element. In addition, the weight element is required to have a weight sufficient to cause shearing force appropriately corresponding to acceleration in order to sense acceleration at a given sensitivity. Thus, the acceleration sensor is heavy. Therefore, it can be concluded that the acceleration sensor of this time has difficulty in realizing lightweight, thin acceleration sensors.
The acceleration sensors of the type in which acceleration is sensed from deflection of the piezoelectric element are required to laminate a plurality of very thin piezoelectric plates in order to cause the piezoelectric element using the laminated plates to be sufficiently deflected in response to acceleration. The laminated structure can be produced only by a very complex process and is thus expensive.
A general object of the present invention is to provide an acceleration sensor in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a compact, thin, less-expensive acceleration sensor having improved sensitivity.
The above objects of the present invention are achieved by an acceleration sensor including a piezoelectric single-plate having a weight part and a detection part. The piezoelectric single-plate is, for example, an X-cut plate of LiNbO3.